A team of Bay Area engineers is helping fast-food giant McDonald’s re-imagine its seasonal mint-chocolate Shamrock Shake with a special straw designed to precisely mix its flavors — and for the lucky few who get one, the experience is sublime.

Until now, McDonald’s hasn’t changed its green confection — which generates proceeds for the Ronald McDonald House — since it first was introduced in the month leading up to St. Patrick’s Day in 1970.

But McDonald’s is mixing it up this year. One of five total iterations of the shake features a brown chocolate half on the bottom topped by a green layer of mint. The presentation, though pleasing, makes you wonder: How do consumers enjoy both flavors at exactly the same time?

With a conventional straw, they would have to either hold it so the end sits precisely between the layers, or move it up and down between the two flavors, explained Scott Rodrick, owner of the Union Square McDonald’s in San Francisco.

That’s a lot of work for something that’s supposed to be fast food.

McDonald’s headquarters decided a new straw, which allows the consumption of both flavors simultaneously in a perfectly even blend, was in order. So 2,000 of these special straws have been distributed randomly nationwide. But the chances of getting your hands on one is akin to getting a golden ticket to Willy Wonka’s chocolate factory.

But its manufacturer, a Santa Cruz-based health care company that did not want to be identified, plans on making more in subsequent years, said Elwin Ong, president of the company that designed the straw.

In the heart of America’s tech capital, McDonald’s stumbled across Ong’s company, JACE.design, a San Francisco-based studio that creates products for the likes of Google and NASA. When McDonald’s asked Ong to make them a new straw, he figured it was a joke.

But the final design — a J-shaped apparatus sporting two holes on its outer side and one on the bottom — took Ong and his team of robotic and aerospace engineers, who partnered with NK Labs in Cambridge, Massachusetts, more planning than they initially had expected. They developed many straws over a weeklong period.

“We went through several rounds of iterations,” Ong explained. “There was a telescoping one and several others, which were much more complicated.”

Their final straw, though much simpler, still “doesn’t look like the straw that I grew up with,” Rodrick said. He thinks it looks like “a piece of mountaineering equipment.” Others say the straw resembles a snorkel.

Ong and his team call it the STRAW, which stands for “Suction Tube for Reverse Axial Withdrawal.” The device relies less on technology than fluid physics, Ong said. Still, he and his team had to perfect every dimension, such as the size of the straw’s holes — each placed precisely so the consumer always sips in fluid, not air — “to optimize the drinking experience,” he said. Indeed, the STRAW delivers a perfectly partitioned chocolaty, minty medley to chocolate Shamrock Shake drinkers.

Today, the straws are being slurped around the country, if they’re not being listed for hundreds of dollars apiece on eBay.

Rodrick is one of the few people who have tried the STRAW — he was invited to the lab where it was being tested.

“As I was walking out, it was kind of like ‘Willy Wonka and the Chocolate Factory,’” he said with a laugh. “They patted me down and actually took the straw back.”

He’s still waiting to call one his own, but Rodrick isn’t too upset: “To have technology here in San Francisco and on the East Coast in Cambridge to collaborate and make good on this 21st century straw has been a lot of fun.”

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